Utilization of wood waste and the like



1941- A. M. THOMSEN 2,228,616

UTILIZATION OF WOOD WASTE AND THE LIKE Filed Feb. 3, 1939 U000? MMZE M FLu/z'd nun: 7 {8005) i020 E) #531 21 Bus-sure fli INVENTOR.

Patented Jan. 14, 1941 UNITED STATES 2,228,616 UTILIZATION OF WOOD WASTE AND THE LIKE Alfred M. Thomson,

San Francisco, Calif.

Application February 3, 1939, Serial No. 254,420

7 Claims.

About the only use for wood waste at the lumber mill is to burn it up and to the extent to which it is thus used as a source of power it is, of course, not a waste in the strict sense of the word. But while the wood now being thrown away in the slab fire and the incinerator constitutes a complete waste, there are two other wastes of even greater importance because of their tremendous volume; these are the waste left in the forest and the amount now being used as fuel in the form of hog-fuel. In the same class I also include such other woody waste as the bagasse from the sugar mill and similar in-' dustrial operations.

Owing to the particularly unfavorable mechanical condition of such wastes they are burnt at very poor efiiciency in boiler fires, yet about the only commercial manner of preparing wood waste I and the like for handling is to tear it .apart in a hog after which it may be conveyed mechanically. The same function is performed in the sugar mill by the mill itself and most other industrial wastes that are now consumed by fire are discarded in a comminuted form and containing much moisture. An efiiciency of little better than 50% is obtained in the boiler with such material, and if power be the aim then the over-all efilciency of the conversion will rarely exceed 6%.

Many suggestions have been made to utilize such material by distillation, for varying amounts of acetic acid, methanol and tar can of course be obtained in this fashion. Few, if any, of such suggestions are in use today for the low price of the products, the low yield and the great dilution of the wood vinegar combine to make a setup that is only of academic interest. Nevertheless, the inherent value of such material is far too great to accept the present method of utlization as final.

I have succeeded in obtaining a very good solution of the difficulty by grouping sundry divers items around the rather high conversion factor of the internal combustion engine. To gasify the waste and use the gas in a gas engine would be obvious and would give a higher conversion factor than that described above, but this has not generally come into use because of the increased complication which is not warranted by the added economy. In addition, the dilution of the gas stream of simple distillation would be so augmented by the enormous amount of gas yielded by the combustion of the charcoal tocarbon monoxide that all hope of securing'any volatile distillate by condensation alone would be.

futile.

In United States Patent 1,839,277, dated Jan. 5, 1932, I have described a method whereby the disadvantage due to such dilution can be overcome, but even this step byitself is not suificient when such poor material as lumber mill waste and sugar cane bagasse are to be used as the starting point. It does, however, find its proper application in the complete exposition of a method of utilizing wood waste where in addition to a very high power efiiciency I also obtain con-; siderable process steam and gather together all other heat wastes in a fluid used cyclically to perform evaporation and/or secondary power. development. 4

To properly understand how this three-fold objective is accomplished it is first necessary to focus attention on the heat cycle of the internal combustion engine. While not absolutely accurate we may consider 30% of the heat content of the fuel as convertible into mechanical energy, the remaining 70% being divided between the heat lost in the exhaust gases and in the cooling water of the cylinder jacket. There isv also a small loss in engine friction, etc. but such small items may be dropped from this discussion. The heat losses'in exhaust and jacket are approximately equal but vary with engine design and performance.

The only plan for utilizing this heat discard from the internal combustion engine of which I have any knowledge-is to pass the hot exhaust gases through a waste heat boiler and make such low pressure steam as this disposal makes possible. Stack temperatures must be placed at a lower limit of 350 F. and as the exhaust rarely exceeds 800 F. it follows that only about /2 of the heat in the exhaust is convertable into steam,

Hot water from the jacket can of course be used to feed this boiler, but it will be obvious that only an exceedingly small amount of the total.

discard can be thus employed. 7

, It will be evident that the chief drawback to any utilization of this waste of heat from the internal combustion engine is the low intensity throughout. I will now mention how a portion of this can be directly converted into fuel for the engine and another portion used to collect other heat losses into a continuous stream of very hot water. This water is then cooled by doing useful work and then cyclically returned to pick up an additional quota of heat. The vacuum evaporator is a most efficient machine for converting this heat into evaporation, a return flow of water erning factor.

cooled to 120 F. being readily obtained. Even better than the evaporator is the device of using this hot Water for power generation by passing it through the countercurrent heat interchanger of an engine utilizing a fluid more volatile than water, such as ammonia or sulphur dioxide, in which case it is possible to cool the water down to about 80 F. before it is returned to the jacket producer with chemical reclamation, an internalcombustion engine with full heat recuperation,

and finally the means for utilizing this heat and that discarded by the gas producer as well.

To convert waste wood into gas in an efficient manner is no simple task for it packs into analmost impervious mass and hence stops all passage of heat through it. It can, however, be converted into a suitable material by screening out the fines, commingling these with a binding medium which will on distillation yield a carbonaceous residue, and then charging the mixture together with the coarse product first separated into a gas producer. In this manner the fines will be present as relatively large masses of damp or wet coherent material interspersed with the coarse material. As the charge descends'in the furnace these lumps or masses Will distil and form coherent aggregates which still further aid in rendering the charge manageable.

The binding material may be any heavy oil or tar or even spent cooking liquor from the sulphite process, provided that'it possesses the required attribute namely, to leave a coherent carbonaceous residue on distillation. In practice, the tar produced in the operation will in general suffice, being converted in use intoligh'ter, recoverable tar oils;

An analysis of wood distillation technique will show the progress of the charge as it. descends in the furnace; Wood waste will contain about 9,000 B. t. u. per pound on a bone dry basis but it will be almost or more than half water in the state in which it enters the furnace. The first step, therefore is drying and will require about 1,000 B.'t. u. per pound of bone dry material. On carbonization it will yield charcoal in accordance with the type of wood charged and in general one-half of fuel value will be found in the charcoal and one-half in the volatile ingredients. Carbonization itself is, of course; strongly exothermic and in the process gives off heat equivalent to about 6% of thetotal, or 540 B. t. u.

The heat required to dry the charge and bring it to carbonization temperature is furnished-by a part of the primary heat ,of combustion of the charcoal near the grate where it burns to carbon monoxide with evolution of approximately .onethird of its B. t. u. content, ,i. e., about 1500 B. t. u. per pound of bone dry charge. This amount, together with that of theexothermic reaction is manifestly far too much for the distilling operation and unless it were decreased, would result in much loss through decomposition of the volatile chemicals set free from the distilling charge in the upper, cooler part of the furnace. I effect this cooling by permitting a part ofthe exhaust gases of the internal. combustion engine to enter with the air of combustion of the gas producer. It seems strange to cool a furnace by adding heated gas to it but in this case the reduction of the carbon dioxide to carbon monoxide by contact. with the incandescent charcoal is the gov- Thegas, being'heated, permits of a correspondingly greater amount being used than would otherwise be the case and this sensible heat is thus salvaged in almost 100% effective manner.

Under favorable conditions it may be possible to return as much as one-half of the exhaust gases in this manner and the conversion factor of the producer-engine unit will be proportionately enhanced. The efficiency of a gas producer is generally placed at 80% which with engine efficiency of 30% makes an over-all factor of but 24%. The reason for the low efficiency of the gas producer is the large amount of heat present as sensible heat in the issuing gas and in latent heat of evaporation of the contained water. How this is recovered and utilized will now be described.

The jacket water of the engine should rarely discharge above 140 F., a temperature too low to do much with it. It can, however, be used to collect other heat wastes from the total assembly and. thus have its temperature raised to an efficient temperature. .That portion of the exhaust of the engine which was not returned to the producer can be thus absorbed and so can all the heat above 212 F., both sensible and latent, resident in the gas issuing from the gas producer. The absorption of the heat in the engine exhaust can conveniently be efiected by merely scrubbing the gas with the jacket water, and this can still be done even if some low pressure steam hasbeen made previously.

Owing to the presence of the recoverable volatiles from wood distillation in the gas from the producer, this simple system is not applicable but in place thereof a countercurrent heat exchanger must be substituted which acts simultaneously as a condenser for the volatiles and as a heater for the jacket water. In this manner all heat Waste of the total engine-producer assembly be,- comes collected in a stream of water at nearly the boiling point. The heat not thus represented has been converted into mechanical power except for the tiny unavoidable losses due to radiation and conduction.

The utilization of the heat thus assembled next requires attention. If the plant at which the operation is performed be one thatinvolves evaporation then a convenient wayto do usefulwork and thus cool the circulating fiuid is to pass it thru the calandrias of a multiple effect evaporator, returning it at about 120 F. to the jacket of the engine. If the evaporator be a triple and if the water enter at 212 F. then the 90 F. abstracted will divide equally between all effects performing the corresponding evaporation in each. In addition, the steam evolved in the first effect is twice compounded in the latter effects and-that r'eleased'in the second effect is compounded once in the third effect. Evaporation is thus performed at a cost of less than 5003. t. u. per pound of water, or the efiiciency of a duplex evaporator; p

If the demand of the plant be for power and 'not for evaporation then a better use for .the

heated circulating fluid is to use i-t'as the heating medium of an engine employing a morevolatile fluid than water. If ammonia be selected, then the liquid ammonia will pass countercurrent to the heated water which thereby becomes cooled to about 80 'F.,- at the same time becoming itself heated to approximately 212 F. At this temperature ammonia has a very high pressure and hence is capable of generating much power as it, is expanded down to the vapor pressure of ammonia at the'te'mperature of the available cooling water.

The lower temperature of this return Water is an added advantage for it permits of better absorption of heat throughout while the heat utilized on each pass is nearly 50% more than if the evaporator be the means employed. From these comments it follows that a combination of bothwith the. commingling of the cooled fluids will present something close to the ideal.

The underlying principle of this method of heat reclamation is of course identical with that of the so-called binary engine which, with ether as the working fluid, was introduced almost a century ago and exists today as the mercury vapor engine, water being in this case the binary fluid. It is therefore unnecessary to describe it more fully. I, of course, do not make any claim to,

originality in this matter, but in all circumstances where it has been applied it has been in such a way that it replaced or detracted from the usefulness of the orthodox condenser assembly. I believe that its application is new and novel as the means of cooling a circulating fluid which in turn assembles waste heat from the jacket and exhaust of an internal combustion engine and from the waste heat of the gas producer as well. For this purpose it is well suited.

Before it is possible to use producer gas made from wood in an internal combustion engine, said gas must be most carefully cleaned from all tarry matters. In addition it must also be freed from acid vapors or else the engine will soon suffer from corrosion. As acetic acid vapor is quite soluble in the gases leaving the producer, even after cooling to normal temperature, it follows that some special method is required to remove it. An

, excellent way of doing this has already been referred to by citing U. S. Patent No. 1,839,277, so no further description is needed.

In giving this description I am aware that I am repeating much that is old. For such, naturally, no claim is made; but there are certain items upon which the increase in efficiency largely depends and these I believe to be original with myself. In order to present my improvements clearly, it has been necessary to use much well known practice but upon the flow sheet I have indicated clearly by witness numbers my own contributions.

Referring then to said flowsheet: The number I has been placed upon the line indicating the return of tar from the tar scrubber as the binder used in agglomerating the fines before they join the general charge going into the gas producer. The number 2 has been placed upon the line indicating the return of a suitable portion of the hot exhaust gases to the producer for the purpose of absorbing a part of the primary heat of combustion. The numbers 3 and 31 have been placed upon the lines indicating the flow of the hot jacket water to the condenser of the chemical division and to the exhaust gas scrubber respectively. The number 4 has been placed where these flows of water join again after having performed their function of absorbing additional heat and before they enter the evaporator and/or engine division, where this absorbed heat is, in part, removed. The number 5 has been placed upon the line which indicates the return of the cooled circulating fluid to the point of lowest intensity in the circuit, to wit: The engine jacket of the internal combustion motor.

I wish it understood, however, that I do not tie myself down slavishly to the actual, flow illustration of the diagram. Thus, the exhaust scrubber and the condenser of the chemical division might be operated in series with the jacket water instead of in parallel as indicated without affecting the validity of my claims. Likewise, I have not indicated, for the sake of clarity, another step which will be needed if anything like one-half of the exhaust is to be returned to the producer. That is to use the portion of the exhaust not so employed to preheat the air of combustion of the gas producer. It will be evident that to thus step up the temperature of the incoming air will increase the temperature of the zone of combustion and thus permit of the use of more exhaust gas to produce the requisite cooling.

As a preferred illustration of the place where my invention belongs in industry I will now describe its use in a newsprint mill. About 75% of the furnish to the newsprint machine is ground wood which requires for its production both power and wood. At present the price of the finished paper is so low that it can only be produced where power is very cheap and wood likewise. This limits its production tocertain favored areas of such cheap power but this aspect is entirely changed if we can use waste wood as an advantageous source of power. The mill needs in addition considerable steam for the drying of the paper and unless it be situated on the open ocean it will require a considerable amount of evaporation to render combustible the spent sulphite cooking liquor from the sulphite portion of the furnish to the paper machine.

It will be seen that all these requirements are met by the herein described heat cycle. The waste wood is made to yield a commercial product in the shape of acetate of soda and also oer-1 tain tar oils all of which are of use as byproducts to decrease the cost of the power. This is done not so much because of these products but because the subsequent use of the gas demands it. The gas engine furnishes normally a cheap type of power and the amount produced per unit of wood is much enhanced by the return of heat from the gas engine to the producer to be returned in the shape of increased chemical energy, Finally, the waste heat not thus utilized is gathered up by the circulating jacket water and converted either into additional power or into evaporation for spent cooking liquor. This again permits us to add the concentrated liquor to the tar and thus use it as supplementary fuel in the producer, spent cooking liquor being very much waste wood in the sense herein designated, and an excellent binder as well as it leaves behind a very coherent coke on distillation.

From the economic standpoint this means that the newsprint mill has been made independent of water power, for a single unit of hog fuel will supply the power needs of a ton of paper while the additional recoveries from the chemical division will still further enhance the situation. Such illustrations could be multiplied with ease.

Having thus described in full my invention and called attention tothe fact that I regard only a definite portion of all this descriptive matter as that which is original with myself, I will now further define this as follows.

I claim:

1. The method of increasing the mechanical efliciency of a gas producer operating upon wood waste which comprises: Separating the charge into two portions one of which shall be relatively coarse and the other relatively fine; commingling the latter with a binder which will leave a coherent carbonaceous residue on distillation charging the resultant'magma together with the coarse portion originally referred to into the producer and operating on the charge as thus constituted. r

2; The method of increasing the thermal efliciency of a gas producer-gas engine assembly which comprises: Returning 'a portion of the engine exhaust to the combustion zone of the producer; collecting the heat resident in the remainder of the exhaust and in the gases issuing from the producer in the heated water issuing from the cooling jacket of the engine; cooling this fluid which has thus become heated to approximately boiling temperature by causing it to imp-art a portion of its heat to the working medium of a binary engine and then returning it as cooling water to the cylinder jacket of the gas engine. I

3., A claim similar to claim 2, with the added step that the exhaust gases not used directly in the gas producer be employed to heat the air of combustion of the latter thus increasing the amount of exhaust gas that can be admitted to the producer; converting the charge as thus constituted into gas; burning said gas in the cylinder of an internal combustion engine; returning a portion of the exhaust gases of said engine to the combustion zone of the gas producer to absorb the surplus heat of combustion above that needed for the operation of the producer; collecting the remainder of the heat wastes of producer and internal combustion engine by causing the water issuing from the cooling jacket to absorb said heat thus increasing its temperature correspondingly; cooling the fluid thus heated by causing it to impart a portion of its heat to the working medium of a binary engine; finally, returning the fluid thus cooled to the cooling jacket of the internal combustion engine to recommence thecycle;

5. A claim similar to claim 4, with the added step that a recovery of acetic acid as a metallic acetate and also tar oils be effected before said gas is burned in the gas engine.

6. The method of increasing the thermal efiiciency of a gas producer-gas engine assembly which comprises: Reducing the heat of the combustion zone of the producer by the admission to said combustion zone of a portion of the exhaust gases of the engine at substantially the same temperature at which they are discarded by the engine; and preheating the air of combustion of said producer by means of the heat resident in the remainder of the exhaust gases not otherwise utilized, thus increasing the amount of exhaust gas that can be directly admitted to the combustion zone.

7."Ihe' method of increasing the thermal eificiency of a gas producer-gas engine assembly which comprises: Dividing the exhaust gases of the engine into two portions; using one such portion to preheat the air of combustion of the producer; commingling the remaining portion of exhaust gas, at substantially the same temperature at which it is discarded by the engine, directly with the heated air of combustion; and admitting the mixture of both to the combustion zone of the producer.

ALFRED M. THOMSEN. 

